The present invention relates to a molecular beam source used for thin-film accumulation or deposition and a method for controlling the amount or volume of a molecular beam, through heating a material to be formed on a film-forming surface of a solid body or matter, such as, a substrate, etc., in the form of a thin-film, by melting, sublimating or evaporating material(s) for forming a thin-film element, i.e., generating evaporated molecules for growing the thin-film(s) upon the surface of the solid body, and it relates, in particular, to a molecular beam source for use in thin-film accumulation, and is suitable for accumulating or depositing an organic thin-film upon a film-forming surface of the solid body, such as, the substrate, etc., continuously for a long time, and a method for controlling an amount or volume of the molecular beam therefrom.
A thin-film deposition apparatus, called a “molecular beam epitaxy apparatus”, comprises a decompressable vacuum chamber, into which a substrate, such as a semiconductor wafer, etc., can be mounted under a high vacuum condition, and heated up to a desired temperature, while providing a molecule supply source cell, such as a Knudsen cell, etc., for example, opposing the film-forming surface thereof. Thus, a material for forming a thin-film element (hereinafter, a thin-film element-forming material), which is put into a crucible of the molecule supply source cell, is heated by means of a heater, to be sublimated, melted or evaporated, so that a molecular beam generated thereby is incident upon the film-forming surface of the substrate, i.e., bringing about the epitaxial growth of the molecules of the thin-film element-forming material on the film-forming surface, thereby forming a film of the thin-film element-forming material.
Such a molecular beam source cell, to be applied in a thin-film deposition or accumulation apparatus, has a crucible made of a material such as PBN (Pyrolithic Boron Nitride), etc., for example, and is highly stable, thermally as well as chemically, so as to receive the thin-film element-forming material therein, and an electric heater is provided surrounding the outside of the crucible to heat the thin-film element-forming material therein, i.e., generating the molecules thereof through sublimation, melting or evaporation of the thin-film element-forming material.
In recent years, attention has been paid to an organic thin-film element, such as, an organic electroluminescence (EL) and/or an organic semiconductor, for example, representatively. Those thin-film elements can be obtained, i.e., the thin-film element-forming material is heated within the vacuum, so as to blast the vapor thereof upon the surface of the substrate, and then it is cooled down to be solidified and bonded thereon. In general, the following method is applied, i.e., the thin-film element-forming material is put into the crucible, which is made of an inorganic material such as PBN, etc., or a material having a high melting-point, such as tungsten, etc., and then the material to be formed as a film is heated by the heater provided around the crucible, thereby generating the vapor thereof to be blasted onto the substrate.
An organic electroluminescence (EL) material, being a representative example of an organic thin-film element, is an element for forming a luminous layer from an organic low-molecular material or an organic high-molecular material, which has EL luminous ability, and attention is paid to, in particular, the characteristics thereof, i.e., as an element of the self-luminous type. In the basic structures thereof, for example, on a film of a hole transportation material, such as triphenyl diamine (TPD) or the like, formed on a hole injection electrode, is laminated or deposited a fluorescent material such as aluminuin-quinolinol complex (Alq3), etc., as the luminous layer and, further thereon is formed an electrode of a metal having a small work function, such as Mg, Li, or Cs, etc., as an electron-injection electrode. However, the materials of the thin-film elements are expensive, in general.
By the way, when building up the thin-film elements, there is also the necessity of the time for exchanging the substrates, on the surfaces of which the thin film should be formed, and for adjusting the position of a mask for blasting the material only onto necessary portions thereof, etc. However, since many of the materials for the organic thin-film element as was mentioned above is sublimated or evaporated at relatively low temperatures, the material is evaporated, even during the time of exchanging the substrates and/or aligning the position of the mask, etc., i.e., there is a drawback that the very expensive material is wasted, uselessly.
Then, as is described in the following Patent Document 1, which will be mentioned below, there is proposed a molecule supply source for use in thin-film deposition of receiving the crucibles therein, each of which is hermetically sealed or closed in the structure, wherein a molecule discharge passage is provided for directing the molecules of the thin-film element material, which are generated within the crucibles, towards the film-forming surface, and further comprising a needle valve for adjusting an amount or volume of molecular supply in the molecule discharge passage.
With the molecule supply source for use in thin-film deposition comprising such the valve mechanism, it is possible to cut or shut off the discharge of the vapor of the material for the thin-film element, which is generated within the crucible, by means of the valve, and in addition thereto, to adjust the amount or volume of the molecular supply by means of the needle valve, as well. For the purpose of forming the thin-film while maintaining a constant film-thickness and quality thereof, it is effective to keep the volume of the molecular supply constant per hour, which is discharged from the molecular supply source.
As a representative material for the organic thin-film elements mentioned above, there is already known a material of the EL thin-film element, and many of these materials for forming thin-film elements are granulated or powdered, and they are put into the crucibles in that condition. Thus, with the heating of it within the crucible by means of the heater, which is provided outside the crucible, the material of the EL thin-film element is heated and sublimated, and is evaporated to be discharged towards the substrate, and thereby deposited on the film-forming surface of the substrate for forming a film thereon.
When the material of the EL thin-film element is sublimated and evaporated within the crucible to be discharged therefrom, the volume is reduced, gradually, of the material of the EL thin-film element remaining within the crucible. Then, since the surface area thereof is also reduced within the crucible, the volume of the sublimation of the material of the EL thin-film element is also reduced, gradually, within the crucible. Accordingly, for the purpose of maintaining the volume of the molecular supply discharged from the molecular supply source for use in the deposition of the thin-films, it is necessary to make a larger opening of the needle valve, i.e., the ratio of the cross-section of the flow passage to that when the valve is fully opened, thereby to maintain the volume of the molecular supply discharged from the molecular supply source for use in the deposition of thin-films.
However, with the adjustment made in the volume of the molecular supply by means of the needle valve, it is finite or has a limit, i.e., when the needle valve is opened fully, then it is impossible to increase the volume of the molecular supply much more than that.
In the Patent Document 2 mentioned below, there are disclosed two (2) controller means, as the adjustment means for adjusting the volume of the molecular supply. One of them is an adjustment means for adjusting the volume of the molecular supply by means of the valve, as was mentioned above. The other one is a controller means depending on the temperature at which the crucible is heated up by means of the heater. However, the latter controller means, depending on the temperature at which the crucible is heated up by means of the heater, is not suitable for adjusting the volume of the molecular supply with any accuracy because the control of the controller is direct and has a time delay.
(Patent Document 1) Japanese Patent Laying-Open No. 2003-95787 (2003); and
(Patent Document 2) Japanese Patent Laying-Open No. Hei 6-80496 (1994).